1. Field of the Invention
The present invention relates to a control circuit for controlling an electron-emitting device. In particular, the present invention relates to a control circuit suitable for controlling so-called carbon nano tubes arranged in an array.
2. Description of Related Art
The production of ever smaller structural sizes on a substrate becomes increasingly important with the development of IC (integrated circuit) technology. One of the basic technologies for producing ICs is lithography where a resist deposited on the entire area of a wafer is exposed using a mask which is opaque or transparent at certain positions. For this, a light beam having a predetermined wavelength is produced, is focused by a special assembly using optical lenses employing the phenomenon of light refraction and is then directed to the wafer to expose the resist deposited there. The resolution obtainable by means of this technology depends on the light wavelength. The smaller the light wavelength, the smaller the structural sizes obtainable. The light wavelength, however, cannot be reduced at will because of optical transmission features of the lenses used for focusing the light. In order to generate ever smaller structures despite this physical limit, technologies must be employed where, for example, electrons instead of light are used for irradiating the resist.
Here, electrons are emitted by a suitable electron-emitting device, are focused suitably and directed to the wafer. Since only a very small resist area (pixel) can be exposed punctually on the basis of a single electron-emitting device, it is of advantage to unite several electron-emitting devices to an array to be able to expose greater resist areas at the same time and selectively so that the desired small structures can be produced faster than when using only one electron-emitting device. If, for example, a cathode-ray tube was used for emitting electrons, it would be obvious for a person skilled in the art that only very coarse structures can be generated with the necessarily great physical dimensions of such areas. This is why electron-emitting devices having small dimensions, such as, for example, carbon nano tubes mentioned above, must be used here.
Carbon nano tubes (CNTs) basically are several mostly cylindrical carbon elements having a diameter of several nanometers, which are arranged on a substrate and have a control electrode. If an electrical control voltage of, for example, several 10V is applied to this control electrode, the CNTs will draw an excitation current. The field formed at the tip of these carbon elements causes a field emission of electrons.
Due to the small dimensions, CNTs are of advantage because an array of electron beams with the help of which the desired small structures can be generated faster compared to a single electron beam, can be produced since a CNT array can irradiate several pixels on a wafer at a certain time.
It is, however, a precondition for obtaining the desired structures using a CNT array for each pixel to be exposed by a precise radiation dose which may neither be exceeded nor fall below a certain value. If the dose is too great, too large a structural point will be generated. If, however, the dose is too small, the required energy dose for exposing the resist will not be obtained, i.e. the resist will not be exposed at this position and no structural point or too small a structural point will result.
Due to the manufacturing problems connected to producing nano structures, a CNT array, however, comprises great deviation as regards the electron dose emitted in the array. The result is that a CNT array, apart from functional CNTs, will also comprise some CNTs emitting too small an electron dose. Additionally, it is conceivable for some of the CNTs arranged in the array to be defect and not to emit any electrons at all. If a CNT array is used for irradiating a wafer, no points at all or too small or too big points will be produced apart from the points having the desired size. Due to these manufacturing problems and, connected thereto, the small resolution obtainable, a usage of CNT arrays—even if desired, as explained above—has not been possible for the purpose of irradiation requiring a high resolution.